Was searching my computer for another file, and ran across this one. No idea where I got it, but at one time I knew a guy that worked/had worked at GM Powertrain a long time in developement, and this may of come from him. Just thought it was interesting, didn't know this info was out there. Very clear from the numbers that slowing your vehicles MPH down will save you gas, even though the weight is so high. As many posts as there are on the subject, looks to me like slowing down is the most effective way to increase economy. /forums/images/graemlins/smile.gif "Here's the Aerodynamic Resistance Demand Horsepower figures for a 1973-87 GM truck at different speeds. Data from GMC Truck Selction Manual. MPH Hp required 10 .2 20 1.5 30 4.9 40 11.7 45 16.6 50 24.8 54 28.7 58 35.6 60 39.35 65 50.1 70 62.6 So, good economy will grow difficult at highish speeds. That said, an axle that keeps the engine operating at ~400 or so below the torque peak (probably 2800 rpm is the peak, so shoot for 2400ish) at the fastest speed you usually drive will help. I'll assume you still have the 9.50 16.5 tires which turn 677 rpm, or similar size equivalents. Lets say you want best economy at 68 mph, which is pretty much the point of no return anyhow... Heres the math; 60/68 = .882 (this is speed correction) 2400/677 = 3.54 (this gives the ideal axle for 60mph) 3.54 x .882 = 3.12 (this corrects the ideal to 68 mph) this suggests the best ratio at 68 mph is ~3.12:1 for this example. A 3.08 axle might work well for this example. Beats me if ones availible however. Perhaps a 3.23 is, which isnt too bad a second choice."